The quality of the bond between the catalyst layer (CL) and the membrane is an important parameter in membrane electrolyte fuel cell technology. The interfacial contact of the CL and the cell membrane has to be continuous to the nanometer scale in order to achieve effective catalyst utilization and to minimize internal cell resistance. The critical importance of the CL-cell membrane interface has been scarcely reported. Pivovar and Kim [J. Electrochem. Soc., 154 (8) B739-B744 (2007)] and Kim et al. [2006 DOE OHFCIT Program Review, May 16, 2006] have presented some details on the crucial significance of the quality of CL-cell membrane interface on the fuel cell performance. In the prior art, Polymer Electrolyte Membrane (PEM) fuel cell technology, the bond between catalyst and membrane is formed relatively readily, typically by hot-pressing a CL/membrane/CL combination or “sandwich”—the so called “CCM” (Catalyst-Coated Membrane). Because the perfluoro-carbon backbone of ionomers used in PEM fuel cells exhibits some thermoplasticity at temperatures below the chemical stability limit, the result of hot-pressing is typically inter-diffusion of the polymer components in the CL and in the surface of the membrane. Such inter-diffusion can generate bonding that can be described as zipping together of micro-fingers of polymeric material protruding from each side of the interface. This form of bonding can secure lasting interfacial adherence in CCMs for PEM fuel cells, typically surviving long term operation at high cell current densities and experiencing significant number of wet-dry cycles.
Wet-dry cycles can be a major challenge to the integrity of the interfacial bond because of the dimensional changes associated with water uptake by the dry polymer material. These dimensional changes can be expected to cause significant stress in the CL/cell membrane interface and could result in gradual delamination that takes place depending, for instance, on: (i) the intrinsic strength of the as-formed interfacial bond and (ii) the dissimilarity of dimensional changes during wet-dry cycles in the materials forming the interfacial bond. In the case of the PEM fuel cell which employs ionomers with perfluoro-carbon backbones, hot pressing under well-optimized pressure and temperature conditions can help to provide a CL/cell membrane interface of good adhesion and of well-matched dimensional changes on both sides of the interface during wet-dry cycles. The strength of the as-formed bond has been confirmed in peel-strength measurements.
In contrast, with ionomers having hydrocarbon, or crosslinked hydrocarbon backbones, such as, for example, in the anion-conducting polymers developed to date, the quality of the CL/membrane interfacial bond formed by hot-pressing a thin film of catalyst/ionomer composite onto the membrane surface, is significantly less satisfactory. One reason is the negligible thermoplasticity of polymers with hydrocarbon backbones. Such polymers with hydrocarbon backbones do not achieve inter-diffusion of ionomeric components across the interface during hot-pressing at relatively low temperatures, for instance at temperatures less than 100° C. Alkaline Membrane Fuel Cells (AMFCs) based on ionomers with hydrocarbon backbones, can therefore suffer delamination at the CL/membrane interface that can become a major cause of performance loss and can lead to complete cell failure. Clearly, the negligible thermoplasticity of the poly[hydrocarbon] ionomers employed in the AMFC membrane and CL calls for alternative methods and structures for securing high quality CL/membrane bonds.
Crosslinking can provide excellent chemical bonding between poly[hydrocarbon] chains. Various crosslinking methods were used in membrane preparation for AMFCs. Xu and Zha [J. Membrane Sci., 199 (2002) 203-210], Park et al. [Macromol. Symp. (2007) 249-250, 174-182] and Robertson et al. [J. Am. Chem. Soc. (2010), 132, 3400-3404] used different diamine compounds to crosslink the polymer in membranes for Alkaline Membrane Fuel Cell (AMFC). Although membranes with crosslinked polymers exhibited excellent mechanical strength, after crosslinking, the membrane surface becomes rigid with very poor surface properties. Similar crosslinking approach within the membrane was applied by Wu et al. [J. Appl. Polymer Sci., 107 (2008) 1865-1871] using UV/thermal curing instead of diamine compounds. Quality of the crosslinked membrane surface, however, did not allow applying a CL on the membrane surface, consequently obtaining inadequate CL-cell membrane-CL interface bond quality.
Similarly to the approach of crosslinking the polymer material in the membrane alone, Varcoe and Slade [Electrochem. Comm., 8 (2006) 839-843] have crosslinked the polymer in the CL alone and mechanically pressed the electrode with such crosslinked CL onto an anion exchange membrane. Similar to other earlier studies of AMFCs, they also obtained poor CL-cell membrane bonding and concluded that inadequate CL-cell membrane interfaces are major limiters of power performance in AMFCs.
In contrast to all those approaches, the present disclosure provides a method of chemically bonding together a CL and an alkaline cell membrane of an AMFC wherein a chemical bond is created across the interface between the CL and the membrane.
While this section of this application is labeled as “Background” Applicants provide this description as information that helps to explain the invention disclosed herein. Unless explicitly stated, Applicant does not concede that anything described in this section, or any other part of this application, is prior art, or was known before the date of conception of the invention described herein.